Bivariate distribution of the ratio between the standard deviation of B_⊥ and its median value, μ, and the brightness temperature spectral index, β, computed in the frequency ranges 115−189 MHz, 467−672 MHz, and 833−1200 MHz (labelled , , and , respectively) for each LOS for the weak and strong field case (left and right panel, respectively), at a resolution of 6.7′. The POSs are identified by the three different colours displayed in the legend. Isodensity contours are plotted at 50 and 80%.